High-frequency amplifier with controlled load impedance



Aug. 3@, 1949.

K. POSTHUMUS 2,480,195 HIGH-FREQUENCY AMPLIFIER WITH CONTROLLED LOADIMPEDANCE Filed May 1, 1946 2 Sheets-Sheet l Aug. 30, 1949. PQSTHUMUS2,480,195

HIGH-FREQUENCY AMPLIFIER WITH CONTROLLED LOAD IMPEDANCE Filed May 1,1946 2 Sheets-Sheet 2 I W r-| I: :l IR. I 0 I51 i A RECT/F/ER l -O C I II 6 V REC TIF/ER Patented Aug. 30, 1949 HIGH-FREQUENCY AMPLIFIER WITHCONTROLLED LOAD GE Klaas' Posthumus, Eindhoven, Netherlands, as-

signor, by mesne assignments, to Hartford National Bank and Trustfiompany, Hartford,

Conn, as trustee Application May 1, 1946, Serial No 666,239 In theNetherlands J auuary 10, 1942 Section 1, Public Law 690; August- 8, 1946Patent expires January 10, 1982 9 Claims. 1

This invention relates to a device for amplifying modulatedhigh-frequency oscillations which comprises at least one amplifier valveand which is especially suitable for the final stage of a transmitter.

For amplifying modulated lush-frequency oseillations it is known tomakeuse: of an amplifier valve whose-grid bias: isso high that duringhalf each cycle of the alternating voltage to be amplified the: valveallows current to pass. Such an amplifier is known as a B-amplifier andat full loadit has an emeiency 01' if the amplitude: of the alternatingvoltage occur-.- ring in the output circuit is assumed to be equal tothe anode direct voltage. In practice, how! ever, is permitted a maximumamplitude of the anode alternating voltage which is equal to from (1.8to 0.9 times the anode direct voltage so that in practice the maximumeificiency. ot a B-amplifier does not exceed from In the use of such anamplifier for amplifying modulated high-frequency oscillations this letficiency' of 67% will alone occur in a positive modulation peak at 106%modulation of the car'- rier wave since only in this case" the amplitudeof the anode alternating voltage is equal to from 0.8 to 0.9 times theanode direct voltage. In the absence of any modulation the amplitude ofthe anode alternating voltage is only half the said value so that theoutput does not exceed 33%. For broadcast stations the mean modulationpercentage over an entire day is very small because the instantaneousvalue of the modulationpercentage seldom vexceeds 30% or in other wordsthe output of a B-amplifier is on the average not much in excess of33%..

In order to obtain an amplifier having a higher output devices have beensuggested in which a load impedance has energy supplied to it by atleast two amplifier channels and in which the output circuit of one ofthese channels is connected to the: load impedance over an impedanceinvertingneuvork, that is to say a network whose input impedance isinversely proportional to the terminal impedance of the output circuit.The impedance inverting network may be constituted by a transmissionline whose length is a quarter of the wavelength of the oscillations tobe amplifled. Such amplifiers permit of obtaining an efficiency of abouteven with low modulation percentage The invention. has for its object toprovide a device for amplifying modulated. high-frequency 0s, cillationswhich has ahighefficiency and in which the use of one amplifier channelsuffices.

According to the invention, the anode circuit oiwhigh-irequencyamplifier valve and a load impedance are interconnected by compoundnetwork whose elements are formed by impedance inverting networks, therebeing arranged at each point-oi connection between two or more of thesenetworks except at the said anode circuit and load impedance at leastone discharge tube whichacts as a switchand whose impedance between theanode and the cathode bridges the input or the Output circuitrespectively of the impedance inverting network at the point ofconnecticn, said discharge tube being so controlled that with increasingamplitude of the oscillatiojns tobe amplified at least part of thesetubes are rendered non-conductive successively and/or a'lternatelyrsothat a stepwise decrease of the input impedance of the compound networkis obtained.

According to one embodiment of the invention the compound network may beobtained by con-. meeting each ot a group of points over impedanceinvertingnetworks to each of a second group of points, the anode circuitof the high-frequency amplifi r valve and the load impedance beingconnected respectively totwo of the said points.

Theemcieney of. a normal B-amplifier is proportional to the amplitude ofthe oscillations to be amplified, At themaximum value thereof theemcieucy is about 67%, at half the value and so forth This is due to the'fact that at the highest amplitude oil the oscillations to beamptified, the anode voltage of the amplifier valve is controlledcompletely but at halt the azaliueonly to the extent of 50% and soforth. The term {complete control of the anode voltest value Ea max ofthe anode alternating voltage.

Ea the output energy a mnx is equal to the maximum value of the energythe tube must be capable of supplying.

In the device according tof the invention the impedance Ra is notconstant but varies stepwise at varying amplitudes of the oscillationsto be amplified. At small amplitudesvRa is higher, for example severaltimes higher than would be the case with a corresponding B-amplifier.This results in the anode voltage Es being completely controlled and theefficiency being high even at a small amplitude. In order that atincreasing amplitude of the oscillations to be amplified increase of theoutput energy may be possible the impedan-ce Ra is decreaseddiscontinuously to a lower amount at which the control of the anodevoltage, and thus the efficiency, fall back slightly but the outputenergy does'not vary. At a further increase of the amplitude of theoscillations to be amplified the output energy may therefore againincrease untilthe complete control of the anode voltage Ea is againattained, the impedance Ra being then again reduced discontinuously andso forth. This procedure may be repeated any number of times until thelast discontinuity causes the impedance Ra to assume the same value asthat which this impedance would possess permanently with thecorresponding B-amplifier.

It is obvious that at amplitudes of the oscillations to be amplifiedbetween zero and the maximum value the mean efficiency is considerablyhigher in the method described as compared with a normal B-amplifier.

In order that the invention may be clearly understood and readilycarried into effect it will now be described more fully with referenceto the accompanying drawings.

The embodiment of the device according to the invention which isillustrated in Fig. 1 comprises a final amplifier valve U of anamplifier for modulated high-frequency oscillations and a load impedanceR, which may be for example an antenna, to which the tube U suppliesenergy.

Between the anode circuit of the valve U and the load impedance R isconnected a compound network whose elements are formed by impedanceinverting networks R0, R1, R2, Rs, R1, R2, R3

R1, R2, and R3", which with the exception of R0 comprise three groupseach of which consists of three networks connected in series and thusforming a circuit which is connected in parallel with the network R0. Atthe points'of connection between the networks R1 and R2, R2 andRs, R1and R2' and so forth are arranged'respectively discharge tubes A and B,A and so forth which act as switches and whose anode circuits are soconnected to the networks that the impedances between anode and cathodeof the tubes A, B, A and so forth respectively bridge the input andoutput circuits respectively of the networks R1, R2, R1 and so forth atthe points of connection, or in other words at the said points ofconnection the networks are terminated by the anode-cathode impedancesof the tubes A, B,

A and so forth. The input impedance of the compound network forms theload impedance Ra: of the valve U, the resistance R constituting theterminal impedance of the network. For the sake of simplicity theconnection lines between the: tubes and networks are shown as singleconductors and the sources of supply and control voltages: are omitted.

The references R0, R1, R2, R1 and so forth designate impedance invertingnetworks, that is to say networks having the property that the in-- putimpedance is inversely proportional to the terminal impedance of theoutput circuit and the output impedance is also inversely proportional.

to the terminal impedance of the input circuit. The product of the inputand output impedance respectively and the terminal impedance is constantand it is hereinafter assumed that for the impedance inverting networkR0 and. R1. R2 and so forth respectively the product of the inputimpedance and the terminal impedance is equal to R02, R12. R22 and soforth respectively. The value R0, R1 and so forth is hereinafterreferred to as surge impedance. In addition for imped ance invertingnetworks, for example for the network R0, the following applies.

in which V designates the voltage across the input terminals of thenetwork R0 and i the current passing through the terminal impedance ofthe output circuit. This current is out of phase with the voltage V. isindependent of the value of the terminal impedance and at a given surgeresistance it consequently only depends on the input voltage V. The sameapplies to the other networks R1, R2. R1, and so forth.

In the device shown in Fig. l the discontinuous reduction of the anodeload impedance Re is obtained as follows: It is assumed that the tubesA, B, A and so forth are so controlled that they are conductive andpossess a low impedance between anode and cathode and thus practicallybring about a short-circuit of the impedance inverting networks at thepoints of connection of these networks where the tubes concerned arearranged. The terminal impedances of the networks R1, R2 and so forthformed by the tubes A, B, A and so forth are consequently very low andthe input impedances of the impedance invertingnetworks at the tube Uand the resistance R are therefore very high, or in other words thenetworks R1, R2. R3, R1, and so forth do not allow the passage ofcurrent. For the input impedance Z1 of the network Ru applies:

RIOS:

Rres is therefore negative if one of the magnitudes R1, R2 and R3 or allthree are positive. The parallel combination of the network Ro with thecircuit Ri, R2 and Rs is equivalenttma new:

w nk-having a surgeresistance that is tosaygthat:thecsurgewresistances Rand Rtes filfi. in 'efiect connected inpara llele Thesurge.

I'ESiSIEE.-11f8f:Rt resulting. fro this; parallelscombianation-isrsmallerwthaniRo if-Rresis positivm Under; ;these.'. conditionsthe; anode. Joad impedance Re .-is .;equa;1: to. the inputimpedance. Zrofnthe; composition of networks Ro-,. R1-.R.2 and Ra,,=.

being smaller than the variation of the anode alternating voltage Es.

of the tube 'U in Fi'g. 1 as a function-"of the am pli-tude' P" ofthe-oscillations tobe ampllfied by the" -t-ube Since the amplificationis substantials ly linear P also represents the amplitude of the currentI that passes through the. output. inn. pedance R1 4 designates themaximumivalue of th oscillationsto be. amplified.and.of...the.. out: putcurrent I and P P2 and P3 :designate the levels at. which the anodelloadaimpedance Ra oi the tube U'is successively. reduced discontinuously the dimensions of the. impedance inverting networks being assumedto be. such .that-the re, sultingsurge resistance RresOf eachotthencirrcuitsRi, R2, R3, R1", R2, Rswand R4", R21, Razz, is equal to Rt. Themaximum' Clll'IEHtT'I" passing through the load resistanceR," that"is'flto' say the current at'coinplete control' of: the anode voltage ojth e tube U is therefore increased;;each"time that one of the circuitsR1; R2, Rzor R ",R",'R.' o 'RflflR", Rs" respectively isadded flby thesame amount Elma: designates the amplitudemf. the anqdeiale tion-"of-theanodeimpedance-Ra 0f the tube dl.

Asexplained before, in the amplitude region between 0 and Piin which thenetworkQRo is alonc switched into the circuit thi impedance-1s In" "theregion-between the, l m 3 1.9 OINPZ and Pi'iol' Ps"a nd .P4 respec fl iR'ir are: connected iinrparallel-z and-the: anode respectively. Fig.2shows how at each discontinuity; the. anode alternating voltage Eefa'llsback from com letecontrol'to a smaller value.

In order to avoid a discontinuity inthe output; it is'n'ecessaury thatdirectly before and after each discontinuous change of the impedance Rethe outputenergyIa2Rzi:I R (Ia designating the alternatin'gcurrent'component of the anode cur-- rent. Qf the-tube U) should besubstantiallythe same.- Since Ra} falls discontinuously'this' is to betakengto 'mean that" Ia must increase discontinuously. Ifpath'e tube UiS'. a triode an increase may occur-can; however, be neutralized byinverse feedback. If the 'tubes A, B, A and "so forth are renderednon-conductive in a manner which is to some extent continuous, thedistortion to be neutralized by inverse feedback is considerablysma1l'r,.than. would. be the case with decisively discontinuousvariations.

"It' isflnotjl essential that the resulting surge resistance of thecircuits R1, R2, R3, R15, R2", R3 and R1", R2", R3" should be equal toR0; that is to say the amplitudelevels .from P1 to P4 are notnecessarily chosen at equal distances. By choosing; ;the elevel 1 P1".to below. .aahighers efiiciencyris obtained,but the..arnounts by whichthe-impedancesuccessiyelydecreases discontinuously at the levels P,i;jP2 and .Bs also become greater, andthis will result-inrarglififit rdistortion; The levels P1 and P3 are preferably so determined by choiceof the networks R1, R2-;:Riiand R1, R2, R3 that the amounts by which-theanode alternating current I must-increase. each time that. one. of, thecircuitsRi, R2,,Rs,.and so forth isadded are equal, the s largest.-discontinuity foccuring, being in. this case a minimum.

lt isiapparent. from the-.ioregoing-thatthe tubes A, B, A and so forthsolely actasswitches, that isgto.,say, that. either. they are :asconductive as possihle;- or -:they. are; entirely non-conductive.The=.tubes. 1A-. and B, Afand B'-and A" andBf respfictivelyqare:rendered non-conductive on-surpassingfihcamplitude level P1, P2 and P3respectiytety 0f? the oscillations tobe amplified, levels at which,asshO-Wn'in FigZ, complete control of the anode, voltage lfl roi thetube U occurs, and in one fomn of? construction of the device accordingto the z inventionz':this.-. non-conductive condition is obtainedzazdueI the fact that a low-frequency voltagfinthatemay benobtained afterrectification and which corresponds with the :modulation .envelopenf:etheuoscillations 'to be amplified :is supplied withinegagtivepolarityto a control electrode of the tubes concerned. The tubes. are so.arranged, for example by means of .difierent grid biasespbtainedthroughresistors H, i2, and .13, that theyarious-sets oftubes, for.example .the tubes.,A.- .a nd. B, A and .13" etc. can berenclerednon-conductive in succession at the desired amplitude levels of theoscillations to be amplinee;- Fl this purpose; the. control :electrodesof the tubes-hand =B;A"-, B and A B" respectively maybe-interconnected;

A further modification of r thedevice accordingfour-circuits"respectively"with-surge =resistancend the invention is'showndn Figs-3. In this -the compound network is obtained due to'thefact that between the anode circuit of the tube U and the load impedanceR is connected a number of parallel branches (two of which are shown inFig. 3) each comprising two impedance invertin networks connected inseries, designated in Fig. 3 R1 and R2, R1 and R2 respectively, at thepoint of connection between which is arranged a discharge tube A, Arespectively that acts as a switch and whose anode-cathode impedanceshunts the input and output circuit respectively of the networksconcerned at the point of connection. The operation essentiallycorresponds with that of the device shown in Fig. 1 except that only onebranch can be operative at a time so that the branches must be possessedof mutually different surge resistances. The fact that it is impossiblefor two branches to be connected in parallel may be recognised asfollows: For a branch comprising two impedance inverting networksconnected in series (for example R1 and R2) applies:

in which Vu designates the voltage across the output terminals of thesecond network and Vi the voltage between the input terminals of thefirst network. For the branch which comprises the networks R3 and R4applies:

If the two branches, that is to say the input and output circuits of thetwo branches, are connected in parallel the voltages Vu and Vi are equalfor the two branches respectively and this is at variance with the twoabove equations if In this case the value Vu=Vi=0 alone satisfies thetwo equations at the same time, or in other words the two branches areshort-circuited on both sides. In the device shown in Fig. 3 connectionof two or more branches in parallel is therefore impossible.

A further modification of the device according to the invention is shownin Fig. 4, the anode circuit of the amplifier valve and the loadresistance R being connected respectively to two opposite corners of ahexagon whose sides and diagonals are formed by impedance invertingnetworks from R1 to R9. At all corners of the hexagon except at the twomentioned before, is arranged a discharge tube designated A, B, C and Drespectively which acts as a switch and shunts the output or inputcircuit respectively of the networks at the point of connectionconcerned.

This device permits of obtaining siX different values of the anode loadimpedance R8 of the tube U by the following combination of conductiveand non-conductive discharge tubes.

IA, B, C, D, non-conductive IIA, B, C, D, conductive I[IA, B conductive,C, D non-conductive IV-A, B non-conductive, C, D conductive V-A, Cconductive, B, D non-conductive VI-A, C non-conductive, B, D conductiveBy reference to Fig. 4 it can be ascertained that with the combinationII the network R9 is alone switched into circuit, whereas with the othercombinations one or more additional circuits .iormed by seriescombination of three of the sesame.

8. other networks are connected in parallel with the network R9. Thetubes from A to D may be rendered non-conductive and again conductive bymeans of a device comprising a cathode ray tube 14 in which the beam ofelectrons is deflected under the influence of a low-frequency voltagethat may be obtained after rectification of the oscillations to beamplified and corresponds with the modulation envelope of theoscillations to be amplified, the beam of electrons with increasingamplitude of this voltage successively striking a number of electrodesconnected respectively to a control electrode of the tubes from A to Din such manner that the tubes from A to D are rendered non-conductive insuccession and/or alternately, for example in the order shown by theabove table.

If only four of the six possible combinations from I to VI are used, towit the combinations from III to V1 the control of the tubes A, B, C, Dcan be obtained by simple means. This may be recognised by reference tothe following table in which the conditions of the tubes A, B, C, D,(b=non-conductive, c=conductive) are indicated at an amplitude of theoscillations to be amplified which increases from zero to maximum. Inthis case the networks from R1 to R9 should be so proportioned that inthe order III, V, IV, VI the combinations made use of lead to asuccessively lower anode load impedance of the tube U in the mannershown in Fig. 2. The amplitude levels at which a discontinuity in theanode load impedance at the tube U successively occurs are desig natedP1, P2, P3.

This table shows that at increasing amplitude of the oscillations to beamplified the tube D or A respectively is rendered non-conductive orconductive respectively at the level P2, whereas the tube C or Brespectively is rendered non-conductive or conductive respectively atthe level P1 and is rendered conductive or non-conductive respectivelyat the level P3.

By supplying a low-frequency voltage, which may be obtained afterrectification of the oscillations to be amplified and which correspondswith the modulation envelope of the oscillations to be amplified,jointly with suitable bias voltages to a control electrode of the tubesA and D it may be readily ensured that at more negative values than agiven instantaneous value of the low-frequency voltage which valuecorresponds with the level P2 the tube A is rendered non-conductive andis conductive at positive instantaneous values and the tube D inversely.The desired control of the tubes B and C can be obtained by rectifyingthe said low-frequency voltage without smoothening and so supplying thedirect voltage pulses obtained to the control electrodes of the tubes Band C that at positive or negative instantaneous amplitudes respectivelyof the low-frequency voltage below a given value corresponding with thelevel P3 or Pi respectively the tube B is conductive and atinstantaneous amplitudes above this limit is non-conductive and the tubeC conversely.

Fig. 5 shows a further modification of the desistance Rrs of thecircuits ofthree series-convice according to the invention whichiisila-rgely similar to'that ShOWH-iHFigi 4. *Inthetablebe low arestatedthe'sixserviceable combinationsof nch-conductive and conductivedischarge=tubes from A to D.

'III-C non-conductive; A, B and D conductive IV-B, C and Dnon-conductive, A conductive 'V--A, C and D'non-c'ondu-ctive', Bconductive VI-A, B and D non-conductive, C conductive In view of thedevicesshownflin Figs. 3, 4 and 5 it may be remarked .thatthe circuitarrangement of the compound networks used in'these devices and built upfrom the various impedance inverting networks andof furthercompound'networks not described which als'omay be employed may beobtained by connecting each point of a group of points over impedanceinverting networks to each point ofa second group of points,

two of the said pointshaving respectivelyconnected to them theanodecircuit-of the :highfrequency amplifier valve'U and the loadim'pedance R. The other points have connected to them the dischargetubesA, B and'C-and soforth.

This is illustrated by Figs. 6a and 61) in whichtwo groups of points$1,112, aa and bi, barbs respectively areconnected by straight lineseach representing' an impedance inverting network. Re-

ferring to Fig. 6a, theanode circuitof thetube Uis connectedto thepoint- (11 and the load impedance Rto the point bi. "If the other points(12, as, be and b3 are assumed to beconnected to discharge tubes thisfigure isiden tical with-Fig. 4.

Similarly, Fig. 6b-is' idntica'1 with-Fig. 5.

Instead of arranging one tube -=A, B, A and so forth respectively'atth'e points of connection between the networksf it ispossibleadvantageous- "1y to arrange two push-pullconnected tubes, no

anode voltage beingrequired for the tubes. A suitablearrangementforthis=is shown in Fig. '7. L1 and L2designateinterconnectedcoils which b'y means of condensers C1and'C2respectivelyare tuned to the frequency of the oscillation tobeamplified. The anodes ofthe'two'pull-push'connected discharge tubes -A1and Aware directly connected to the ends of coil L2. Thecontrolelectrodes of thetu-bes A1 "and Aaareinterconnected and consequentlyreceive "the same controlvoltage. The input terminals'k are connected tothe input or output terminals respectivelyof'theimpedance invertingnetwork at the point" of connection between the networks. When the-tubesA1 and A2 are non-conductive it is'easy to ascertain that the impedancebetween the terminals 70 has a very low value, whereas the'impedance-ishigh when the-tubes A1 and A2 are conductive.

As mentioned before;'- the resulting surge -re- 'nected networks,such'forexampleas the circuits R1, R2 and. Rs used in thedevices-shownin Figs. 1 and 4, must be positive; this may be 'ensured'bychoosing one of the networks-R1, R2 or Rsorall three to be negativethatisto say building them up in such manner that'the outputcurrent' i-is 90out of phase with the voltage Vand hence is opposite to that ofthe'networkRo. A favourable form ofconstruction of thesecircuitsis'obtained by using for the networkRz afilter network whichbrings about a' phasefdisplaceme'nt opposite to that ofthenetwork-Ro, R1being-equal "to R2=R3. The circuit arrangementof this is R2 31theitidtlCtane 1i and "the-"capacity C RIO whichareccrinectedisirriparallel with ithe'tube": A "and B esp activelyformxian oscillatory' circuit which: is "tuned'to the oscillation to. beamplified andwhich'has a constanthigh impedance. This 5 "circuit-maytherefore bexomittedsoi that the simple circuit -arrangement shown inFig. Bb-is obtained. Fig. 9*shows a circuit diagram ofthe modificationof 'the' devi'ce according to Ethe-invention shown inFig; 1, thenetworks fromRi to R3 l0 bing all chosen tobe'identical and built up inthe mannershown-in Fig. 8b. Inste'ad of-usi'ng impedance invertingnetworks built u'p from concentrated inductancesand fca-paei tiesmse-may- 'also' be made-(of impedance 1. ln'verting networks havingadistributed inductance and capacity} for example transmission-lines-havi-ng-alength equal to an odd number cf huarter wavelengths 'ofthecSciIIations to be amplified. As an alternative. each impedance ing'vrting network may be constituted by itself by an-odd numbero-fseries-connected impedance inverting networks.

I claim:

1. Apparatus for amplifying amplitude-modue lated. highfrequency-oscillations comprising an "electron discharge tube providedwith a cathode, a-g'rid' and an anode and circuits therefor theoscillations to be am-plified beingapplied-to'said "grid, a-loadinember, -impedance network means arrangedto couplethe anode circuit ofsaidtube tosaid member,-- m"eans to adjust the impedance of said networkmeans in a-step-wise manner, the input impedance of said network meansform- "ing the anode impedance ofsaid tube and the '35 'load'memberconstituting the terminalimpedance'of said networkmeans, and controlmeans responsive to the varyingxamplitude of said oscillations andcoupled to said impedance adjusting'means to adjust the impedance ofsaid netgcwork in the inverse direction whereby an increase in theamplitude-froma minimum to a-maximum value results in a step-wisereduction'in the anode impedance-of said tube.

' 2. "Apparatus" for amplifying amplitude-modu- 3 'lated high frequencyoscillations comprising an electron discharge tube provided with acathode, a grid and an anode and circuits therefor; a "load'member, acompound network arranged to couple the anode c-ircuit of said tube tosaid member, the input impedance of said network forming the anodeimpedance of said tube and the load member constituting the terminalimpedance of said network, said compound network including a singleimpedance inverting net- '55 Work and a plurality of threeseries-connected impedance inverting networks all connected in parallelwith said single network, a pair 'of normally closed switching devicesfor each of said three series-connected impedances, the devices of eachpair being bridged across the two junctions "in said threeseries-connected networks, and control means responsive to the varyingamplitude of said-oscillations for successively actuating each pair ofswitching devices whereby an increase in the amplitude from a minimum toa maximum 'value effects a stepwise reduction in the anode impedance ofsaid tube. 3. Apparatus for amplifying in a class B man- 'neramplitude-modulated high frequency oscillatlons comprising an electrondischarge tube pro- Vided with a cathode, a'grid and an anode and-circuits therefor, a loadmember, a compound network arrangedtocouplethe'anode circuit of said "tube "to said member, the inputimpedance--01 "said "compound network forming the "anode impedance ofsaid tube and the load member constituting the terminal impedance ofsaid compound network, said compound network including a singleimpedance inverting network and a plurality of three series-connectedimpedance inverting networks all connected in parallel with said singlenetwork, a pair of grid-controlled electron discharge devices for eachof said three series-connected impedances, the devices being bridgedacross the two junctions in said three series-connected networks andbeing arranged to be normally conductive, and control means responsiveto the varying amplitude of said oscillations to apply a voltage to thegrids of said discharge devices to successively render each pair ofdischarge devices non-conductive, whereby an increase in the ampltiudefrom a minimum to a maximum value effects a step-wise reduction in theanode impedance of said tube.

4. Apparatus for amplifying in a class B manner amplitude-modulated highfrequency oscillations comprising an electron discharge tube providedwith a cathode, a grid and an anode and circuits therefor, a loadmember, a compound network arranged to couple the anode circuit of .saidtube to said member, the input impedance of said compound networkforming the anode impedance of said tube and the load memberconstituting the terminal impedance of said compound network, saidcompound network including a single impedance inverting network and aplurality of three series-connected impedance inverting networks allconnected in parallel with said single network, a pair of normallyconductive grid-controlled electron discharge devices for each of saidthree series-connected impedances, the devices being bridged across thetwo junctions in said three series-connected networks, grid bias meansfor said discharge devices, rectifier means for deriving a low frequencycontrol voltage from said tube which varies in accordance with theamplitude of the oscillations therein, and means to apply said controlvoltage to the control grids of said devices, the biases on therespective control grids of said devices having values at which eachpair of said discharge devices is successively rendered non-conductivewhereby an increase in the amplitude from a minimum to a maximum valueeffects a step-wise reduction in the anode impedance of said tube.

5. Apparatus for amplifying in a class B manner amplitude-modulated highfrequency oscillations comprising an electron discharge tube providedwith a cathode, a grid and an anode and circuits therefor, a loadmember, a compound network arranged to couple the anode circuit of saidtube to said member, the input impedance of said compound networkforming the anode impedance of said tube and the load memberconstituting the terminal impedance of said compound network, saidcompound network including a single impedance inverting network and aplurality of three series-connected impedance inverting networks allconnected in parallel with said single network, a pair ofgrid-controlled electron discharge devices for each of said three seriesconnected impedances, the devices being bridged across the two junctionsin said three series-connected networks, grid bias means for saiddischarge devices, rectifier means for deriving a low frequency controlvoltage from said tube which varies in accordance with the amplitude ofthe oscillations therein, means to apply said control voltage to thecontrol grids of said devices, the biases on the respective controlgrids of said devices having values at which each pair of said dischargedevices is successively rendered non-conductive whereby an increase inthe amplitude from a minimum to a maximum value effects a step-wisereduction in the anode impedance of said tube, and a negative feedbackcircuit coupled between the output of said compound network and the gridcircuit of said tube and arranged to compensate for distortionintroduced'by the step-Wise variation in said anode impedance.

6. Apparatus for amplifying amplitude-modulated high frequencyoscillations comprising an electron discharge tube provided with acathode, a grid and an anode and circuits therefor, a load member, acompound network arranged to couple the anode circuit of said tube tosaid member, the input impedance of said compound network forming theanode impedance of said tube and the load member constituting theterminal impedance of said compound network, said compound networkincluding a single impedance inverting network and a plurality of threeseriesconnected impedance inverting networks all connected in parallelwith said single network, a pair of grid-controlled electron dischargedevices for each of said three series-connected impedances, the devicesbeing bridged across the two junctions in said three series-connectednetworks, grid bias means for said discharge devices, rectifier meansfor deriving a low frequency control voltage from said tube which variesin accordance with the amplitude of the oscillations therein, means toapply said control voltage to the control grids of said devices, thebiases on the respective control grids of said devices having values atwhich each pair of said discharge devices is successively renderednon-conductive whereby an increase in the amplitude from a minimum to amaximum value efiects a stepwise reduction in the anode impedance ofsaid tube, a negative feedback circuit coupled between the output ofsaid compound network and the grid circuit of said tube and arranged tocompensate for distortion introduced by the stepwise variation in saidanode impedance, the surge impedances of the respective networksconstituting said compound network having values at which the variationin said anode impedance is effected in uniform steps.

'7. Apparatus for amplifying amplitude-modulated high frequencyoscillations comprising an electron discharge tube provided with acathode,

a grid and an anode and circuits therefor, a load member, a compoundnetwork arranged to couple the anode circuit of said tube to saidmember,

the input impedance of said network forming the anode impedance of saidtube and the load member constituting the terminal impedance of saidcompound network, said compound network including a plurality of pairsof series-connected impedance inverting networks all connected inparallel, the pairs of said series-connected networks havingprogressively diiferent surge impedances, a normally closed switchingdevice bridged across the junction in each pair of seriesconnectednetworks, and control means responsive .to the varying amplitude of theoscillations for successively actuating said devices, only one of saiddevices being actuated at any one time,

- whereby an increase in the amplitude from a minimum to a maximum valueeffects a stepwise reduction in the anode impedance of said tube.

8, Apparatus for amplifying amplitude-modu- 13 lated high frequencyoscillations comprising an electron discharge tube having a cathode, agrid and an anode and circuits therefor, a load memher, a compoundnetwork arranged to couple the anode circuit of said tube to saidmember, the input circuit of said compound network forming the anodeimpedance of said tube and the load member constituting the terminalimpedance of said compound network, said compound network including sixserially connected impedance inverting networks arranged to define anhexagon and three impedance inverting networks connected between pairsof opposing vertices of the hexagon one pair of opposing verticesproviding the input and output terminals of said compound networks, fourgrid-controlled electron discharge devices bridged respectively acrossthe remaining vertices of said hexagon, and control means responsive tothe varying amplitude of the oscillations to render said electrondischarge devices 20 Number non-conductive in an order effecting astep-wise reduction in the anode impedance as the amplitude goes from a,minimum to a maximum value.

9. An arrangement as set forth in claim 8 wherein said control meanscomprises rectifier means for deriving a low frequency control voltagefrom said tube which varies according to the amplitude of saidoscillations, a cathode-ray tube having deflecting means and a pluralityof electrodes arranged to be scanned successively by the beam, means toapply said control voltage to said deflection means to successively scansaid electrodes in accordance with the amplitude thereof, and meansconnecting said electrodes respectively to the grids of said electrondischarge devices.

KLASS POSTHUMUS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Name Date 2,237,796 Smith Apr. 8, 1941 2,255,476Thomas et al Sept. 9, 1941 2,321,269 Artzt June 8, 1943

